Digital sampling laboratory wattmeter

Abstract

where Vi and I, are simultaneous samples of voltage and current equally spaced in time. It has been shown that, provided certain conditions are satisfied, (2) gives an ac- II. THE DIGITAL SAMPLING TECHNIQUEThe average power measured over an interval T is given by Abstract-A versatile laboratory multifunction wattmeter based on digital sampling has been developed for incorporation into a laboratory calibration system. The meter uses two IS-bit AID converters and achieves an overall power measurement accuracy of 0.015 percent of applied volt-amperes at mains frequencies over. the range 100 rnA to 100 A and 5-100 V. Although intended for system use through the IEEE 488 bus, the meter can be used as a stand-alone instrument if required. curate result for relatively few samples [2]. Furthermore, the samples need not be taken over a single period but can be spread over m periods (where m is an integer) allowing the sampling interval to be increased accordingly. The conditions required are: 1) the sampling must occur over an integral number of waveforms; and 2) the waveform is stationary for the duration of measurement.With these two conditions satisfied the result is accurate for all power harmonics except; those harmonics k, for which km / n is an integer. The harmonic power terms arise from harmonics V p and I q in the voltage and current waveforms combining to produce power components at the (p + q )th and (p -q )th harmonics. Therefore if m and n have no factors in common, errors are limited to those voltage-current products where p + q or p -q is an integral multiple of n, the number of samples taken.Hence, for low values of n, errors may not be negligible particularly if both waveforms are nonsinusoidal or if a dc component is present on one waveform while the other contains significant distortion. Nevertheless the technique promises high accuracy for waveforms which are essentially sinusoidal, and it is under these conditions that the power calibration standard is used. III . WATTMETER CONFIGURATIONThe basic configuration of the digital sampling wattmeter is shown in Fig. 1. The instrument is controlled by an 8085-based single-board computer which is multibus compatible. Signal conditioning boards for current and voltage are placed in a separately screened compartment to shield against the noise generated by the processor board. These boards comprise a selectable gain voltage amplifier and current-to-voltage converter for the voltage and current signals, respectively. The gains are automatically switched by the processor so that the output signal levels are as close as possible to, but do not exceed, the ± 5-V input range of the analog-to-digital converters. The digitization is performed by two IS-bit hybrid converters which are simultaneously triggered at a sampling interval previously determined by the processor. The samples from each converter are multiplied together and the products are summed and then averaged to obtain the power measurement. During data acquisition and subsequent processing, checks are mad...